1. Field of the Invention
The present invention generally relates to techniques that allow significant compaction of the data used to describe a design which lacks repetition and, more particularly, to an effective data compaction technique for the generation of complex computer images. The invention has particular application in electron beam lithography for the fabrication of complex integrated circuit structures.
2. Description of the Prior Art
The amount of data required to describe the patterns to be written by an electron beam (e-beam) lithography system for complex integrated circuit (IC) structures such as advanced computer chips, read only storage (ROS), programmable logic arrays (PLAs), and other array structures, is enormous, typically requiring gigabytes (GBs) of data. Generally, the pattern data is not compacted because the patterns lack the repetition needed by step-and-repeat, macro commands and other known compaction methods. This results in excessive data volumes and post processing times for such designs.
R. D. Moore and P. M. Ryan in "Personalization of ROS Memories Using an E-Beam System", IBM Technical Disclosure Bulletin, vol. 20, no. 2., July 1977, pp. 588, 589, describe a method for improving writing speed for a rigidly defined ROS array where the exposure locations and pattern (single spot) and pattern length are fixed and the exposure data is not compacted.
U.S. Pat. No. 4,291,231 to Kawashima et al. for "Electron Beam Exposure System and Apparatus for Carrying Out the Same" describes a system using fixed patterns stored in memory to provide repeated exposure of the same pattern sequence at various locations. The pattern consists of the same sequence of pattern elements repeated over and over. It does not allow individual selection of pattern elements.
U.S. Pat. No. 4,259,724 to Sugiyama, like the patent to Kawashima et al., allows step and repeat exposures of a pattern segment (i.e., a fixed string of shapes) with an elementary repetitive pattern in a rectangular area which may consist of a number of fractional patterns.
U.S. Pat. No. 4,531,191 to Koyama provides for monitoring of the input data string to detect repeated data sequences and generates a repeat count used during actual exposure. In the Koyama system, repeated sequences must occur in succession.
U.S. Pat. No. 4,051,457 to Inose et al. discloses a system which increases the speed by which codes for Chinese characters are interpreted and the address for the string of shapes needed to write the character is obtained.
None of the foregoing patents disclose a system that would allow variations of the basic pattern at each exposure location; that is, they do not allow for selective exposure of shapes within a string.